The present invention relates to glazings and more particularly glazings comprising soldered electrical connections between electrically conductive components on surfaces of the glazings and electrical connectors. The invention also relates to electrical connectors for use in such glazings and to solders to solder connectors to electrically conductive components on the surface of glazings.
Glazings, especially vehicle glazings, may have electrically conductive components, such as circuits printed on the surface of a ply of glass or an array of wires fixed within a laminated glazing, in electrical connection to the wiring harness of a building or, more usually, a vehicle. Such circuits find use as heating circuits, to promote de-misting or de-icing, or as antenna circuits. Generally, electrical connection is made by a connector being soldered to an electrically conductive substrate known as a bus bar, which may be provided directly on the surface of a piece of glass, or fully or partly on a fired, printed band on the glass, known as an obscuration band. The bus bar is typically printed using a silver-containing ink. Historically, the solder used to join the bus bar and the connector contained lead. However, lead is known to be harmful, and there is increasing legislative pressure to use lead-free solders in industry.
Lead-free solders have been disclosed in, for example, WO-A-2004/068643 which relates to tin-based solders (up to 90% by weight tin) comprising a mechanical stress modifier selected from bismuth, indium or antimony. The solder may also contain silver and/or copper.
EP-2 177 305 discloses a lead-free solder alloy which can be used for soldering vehicle mounted electronic circuits, the alloy consists essentially of silver, indium (at 3 to 5.5 mass %), copper, optionally bismuth with the balance being tin.
WO-A-2007/110612 discloses some improved electrical connectors for use with glazings. The structure of the connector is chosen to maximise the adhesion between the electrically conductive components in the glazing and is especially for use with lead-free solders.
WO-A-2007/021326 discloses a solder composition having a mixture of elements including tin, indium, silver and bismuth and which includes between 30 to 85% tin and about 15 to 65% indium.
Unfortunately a number of lead-free solders can introduce problems when used, in particular, in vehicle glazings because such solders are not generally intended for use on glazings and so are not tested by accelerated aging for extended periods nor by extensive chemical testing using aggressive chemicals. More particularly, accelerated durability testing by the Applicants of indium containing solders used on glazings has identified deterioration in performance especially at high temperatures or in humidity exposure tests and thermal cycling tests. The consequence of this deterioration is a reduction in bond strength between the solder and either the electrical connector or the electrically conductive circuit printed on the surface of a ply of glass. Although it is not yet clear what the mechanism is of such deterioration in bond strength, analysis has suggested that the formation of an intermetallic layer at the interface between the solder and the connector (especially when the connector is copper-containing) and also at the interface between the solder and the electrically conductive circuit (especially when the circuit is printed using a silver-containing ink) may be responsible. Cracks seem to form at these interfaces after thermal and/or humidity cycling.
The intermetallic layer at the interface between the solder and the connector is thought (at present) to result from a reaction between components of the solder (e.g. indium) and components of the connector (e.g. copper). The formation of intermetallic layers has been also suggested in Application Notes from the Indium Corporation of America (e.g. Indium/Copper Intermetallics).